This invention relates to an assembly comprising first and second components which are retained together by a retaining element.
It is known to use a split pin, or cotter pin, to retain first and second components together, for example to retain a clevis pin against axial displacement within a retaining ring. The cotter pin is inserted through aligned holes in the retaining ring and the clevis pin, and the protruding tangs are bent outwardly from each other to prevent withdrawal of the cotter pin. Typically, the tangs are bent fully round into close engagement with the outer surface of the retaining ring to ensure the best locking function, to minimise fretting, and to leave a neat assembly less prone to snag.
Bending the tangs requires an additional fitting operation after the pin has been inserted. Disassembly is not always easy, and can cause damage to the components.
Shape memory effect materials are known. Components made from such materials exhibit the property of returning to a predetermined “memorised” shape when their temperature changes through a transition temperature. Typically, the component resumes the memorised shape when heated from the “cold” state above the transition temperature to the “hot” state.
A known shape memory effect material is Nitinol, for which the transition temperature may fall in a range extending from below 0° C. to above 150° C. In the “cold” phase, ie below the transition temperature, Nitinol has a martensitic structure, whereas in the “hot” phase above the transition temperature it transforms to an austenitic structure. The memorised shape is fixed by forming the component to the desired shape and then heating it, while maintaining the shape, to an elevated temperature (for example about 500° C.). Subsequently, when the component is reduced in temperature to below its transition temperature, it transforms to the martensitic structure, in which form it has a relatively low Young's modulus and can be deformed under moderate stress. Thus, the component can be formed into a first configuration in the “cold” state. If the component is reheated to the “hot” state, above the transition temperature, it reverts to the austenitic structure and to the previously memorised shape, constituting a second configuration. The transformation results in an increased Young's modulus, so that the second shape is strongly resistant to deformation.
If the component is then cooled again, below the transition temperature, the memorised shape is retained unless the component is subjected to a stress sufficient to deform it. The cycle can be repeated many times, with the component reverting to its memorised shape each time it is heated above the transition temperature, even if it is deformed while in the “cold” state.
Although many materials exhibiting shape memory effect are metallic alloys such as Nitinol, some polymers have also been developed which exhibit the effect. Also, some materials operate in a two-way manner, in that they can have two memorised states, one of which is adopted at temperatures above the transition temperature, and the other of which is adopted at temperatures below the transition temperature. Also, some materials have a single “memorised” state which occurs in the “cold” phase, below the transition temperature, so that the component will resume a predetermined shape at low temperatures after deformation occurring at higher temperatures.
U.S. Pat. No. 5,791,899 discloses the use of a shape memory effect material in a bone anchoring assembly in which a coupling member of shape memory effect material is of a size to be movable within a cavity in a bone anchor while in the “cold” state, but expands or deforms in the memorised state above the transition temperature to become secured firmly within the cavity.